Implement `&&` and `||` Operators

Resolves issue #227 by implementing support for && (AND) and || (OR) operators. There's also basic background support, so background jobs will run in the background and print when they are complete.

The following command:

echo one && echo two && echo three

Will print

one
two
three

While the following:

echo one && echo two || echo one

Will print

one
two

And the following:

ecsdf one && echo two && echo three || echo four

Will print

ion: command not found: ecsdf
four